David Silverman, the president of American Atheists and the man behind the billboard, said it would remain in place at least until the winter solstice on Dec. 21 and possibly through Christmas. He said the billboard was partly inspired by one that American Atheists’ founder, Madalyn Murray O’Hair, set up in Dallas in the 1970s proclaiming “Atheism: It’s not what you believe.”

Mr. Silverman said the billboard served two purposes. The first was to get the many people who do not actually believe in God but practice religious rituals to “come out,” in his words.

He said the billboard’s location was especially effective because commuters “drive by this sign very slowly every day for a month, right in the Christmas season.”

“And when they go into New York to go shopping,” he said, “they’re going to see it.”

Ironically, in his desire to out Christians who are just going through seasonal retail motions, his billboard may serve to remind believing Christians of the real reason for the season.